Electric power devices used in a substation include, such as a gas insulated circuit breaker, a gas insulated switch gear, a gas insulated bus bar, a gas insulated transformer and the like. These electrical power devices are gas insulated apparatuses which include a high voltage conductor or conductors supported by insulation members accommodated in a hermetically sealed metallic vessel filled with insulation gas.
In such a gas insulated apparatus, if an internal defect such as a metallic extraneous substance exists inside the metallic vessel, the intensity of the electric field inside the vessel becomes very high locally, and a partial discharge may occur therein. Such partial discharge may result in a dielectric breakdown, if its occurrence is left as it is. Further, there is a fear that such dielectric breakdown finally causes a serious accident. Accordingly, it is necessary to take some countermeasures by detecting these partial discharges occurring in the metallic vessel at the early stage thereof, and it was known that a possible dielectric breakdown can be predicted in advance by detecting the partial discharge signals, and the partial discharge detection was considered important as a method of preventive maintenance of such gas insulated apparatuses.
The partial discharge occurring in an atmosphere of SF.sub.6 gas generates a current in a sharp pulse form, whereby electromagnetic waves induced by the partial discharge are radiated in a moment in a space inside the metallic vessel and are propagated inside the metallic vessel. Therefore, an insulation abnormality can be predicted by detecting these electromagnetic waves. However, the metallic vessel is hermetically sealed, the electromagnetic waves are scarcely leaked outside the sealed metallic vessel. Accordingly, the following partial discharge detectors for a gas insulated apparatus have been proposed which detect signals inside the sealed metallic vessel and the detected signals are transmitted to an external measurement unit via a sealed terminal, for example, JP-A-3-56016 discloses a provision of a loop antenna inside the metallic vessel for the gas insulated apparatus, and detection by the loop antenna of the radiated electromagnetic waves induced by the generated partial discharge, and further, JP-A-3-164023 discloses, a method of detecting the electromagnetic waves radiated with a slot antenna from outside the metallic vessel.
Further, in a gas insulated apparatus, it is necessary to carry out a measurement of an operating voltage to investigate whether or not a normal voltage is applied, as well as a measurement of a surge voltage to investigate a surge caused by a lightning or a switching operation of various switches. A conventional gas insulated apparatus was provided with a voltage detector capable of detecting such voltages as mentioned above.
For such a voltage detector provided in a gas insulated apparatus, the detector as disclosed in JP-A-8-248065 has been known so far. The voltage detector is of a non-contact type and detects the voltage by an electrostatic voltage dividing. In the conventional partial discharge detection methods for the gas insulated apparatus as has been explained above, the partial discharge signals were very feeble and were likely to be influenced by external noises. Therefore, the detection sensitivity and accuracy of the partial discharge signals were not necessarily satisfactory, and a highly sensitive partial discharge detection method is desired. Further, since the partial discharge occurring position varies, it is possible that not only the magnitude of the signal varies but also the output with respect to frequencies varies largely. Therefore, in order to locate the partial discharge occurring position a highly sensitive detection is required over a broad frequency band over 0.about. to several GHz.
Further, when an accurate voltage measurement was required, the size of the measurement device increases. Therefore, a simple but accurate voltage measurement method is desired.
An object of the present invention is to provide a partial discharge detector for a gas insulated apparatus which shows a high detection sensitivity over a wide range of frequencies.
Another object of the present invention is to provide a partial discharge detector for a gas insulated apparatus which permits measurement of an operating voltage and a surge voltage.
Still another object of the present invention is to provide a partial discharge detector for a gas insulated apparatus which shows a high detection sensitivity over a wide range of frequencies and permits measurement of an operating voltage and a surge voltage.